A conventional example of a water treatment apparatus which filters clear water, sewage, nightsoil, industrial waste water, or the like through a membrane is shown in FIG. 4. As shown in FIG. 4, the submerged membrane separation apparatus has a separation membrane module 2 immersed in a treatment tank 1 in which a plurality of plate-like filtration membranes (membrane elements) are arranged in parallel and a treated water piping 3 that is communicated with the transmission side of the separation membrane module 2. Further, an air diffusing apparatus 4 is arranged below the separation membrane module 2 and a gas supplying apparatus 7 for supplying gas is provided via a gas supplying pipe 5. A suction pump 8 is used as a driving force of filtration. The liquid to be treated in the treatment tank is filtered through the separation membrane module 2. The filtrate is taken out of the treatment tank through the treated water piping 3.
Here, when filtration is operated, the stable filtration operation is performed in the manner that gas is supplied from the gas supplying apparatus 7 to the air diffusing apparatus 4 via the gas supplying pipe 5, gas is ejected from an air diffusing hole of the air diffusing apparatus 4 into the treatment tank 1, a gas-liquid mixing upflow caused by air lift action of the gas being ejected is made acted as traction on the membrane surface of the filtration membrane, the adhesion of a cake layer to the membrane surface is suppressed while the filtration is carried out.
In this case, a phenomenon that the liquid to be treated is flowed from the air diffusing hole during gas supply into the apparatus is often observed in the air diffusing apparatus 4. The air diffusing hole is clogged with solids which are formed when the liquid to be treated is dried by gas. As a result, the gas supply from the air diffusing apparatus is insufficient and a poor cleaning of the membrane surface is caused by insufficient traction.
Therefore, as a technique of preventing obstruction of the air diffusing hole, proposed is an apparatus in which an air diffusing apparatus has a structure formed only of a main pipe without a branched structure, gas supplying pipes are respectively connected to both ends of the main pipe of the air diffusing apparatus, and valves are placed on one or both of the gas supplying pipes (Patent document 1). However, in the air diffusing apparatus, a separating effect of solids which are once adhered to the air diffusing hole is not sufficiently obtained only by changing the gas flow direction by opening and closing of the valves, which results in difficulty in maintaining the performance of the air diffusing apparatus. This similarly occurs when a cleaning liquid is injected into the air diffusing apparatus and when a branched pipe that is openable and closable is provided.
Further, as another technique of preventing obstruction of the air diffusing hole, proposed is an apparatus in which an air diffusing apparatus is connected to a gas supplying apparatus via an air supply pipe, a pressure reduction means which reduces the internal pressure of the stem pipe to the level lower than the liquid pressure around the outside of the air diffusing apparatus is provided, the pressure in the stem pipe is reduced by operating the pressure reduction means when all of the gas supplying apparatuses are stopped, and a control unit that stops the process of reducing the pressure in the stem pipe by the pressure reduction means when at least any one of the gas supplying apparatuses is driven is provided (Patent document 2). However, in the air diffusing apparatus, solids separated from the air diffusing hole are again passed through the air diffusing hole and discharged to the outside. Thus, solids block the air diffusing hole again at the time of discharge. Further, the gas supplying apparatus tends to be larger in larger-scale treatment facilities. It is difficult to stop and re-drive the gas supplying apparatus frequently from the viewpoint of equipment operation management.
Further, as another technique of preventing obstruction of the air diffusing hole, proposed is an apparatus in which an air diffusing apparatus is communicated, at its base end side, with a gas supplying apparatus which is an air supply source, the tip side is communicated with an air diffusing drain pipe, the air diffusing drain pipe is open as an exhaust port in the upper position of the air diffusing apparatus, and a drain valve is provided (Patent document 3). However, the air diffusing apparatus supplies air from only one direction and thus solids blocking the inside can be discharged to only one direction. Therefore, even if solids adhered to the air diffusing hole can be removed, they can be remained and blocked again in the air diffusing drain pipe. In this case, the cleaning cannot be further performed.
Further, as another technique of preventing obstruction of the air diffusing hole, proposed is an apparatus in which an air diffusing apparatus is composed of a plurality of fine bubble diffusing pipes and a plurality of gas supplying pipes for supplying gas to the fine bubble diffusing pipes and the plurality of gas supplying pipes are arranged so as to be opposed across a vertically lower part of a separation membrane module (Patent document 4). However, the air diffusing apparatus does not have a mechanism for cleaning the inside of the apparatus, for example, a drain valve or a valve that changes the flow direction. Therefore, when solids are accumulated in the apparatus by any cause, the apparatus needs to be disassembled to remove them. This leads to a very complicated operation.
Prior Art Documents
Patent Documents
    Patent document 1: Japanese Patent Application Laid-Open (JP-A) No. 2006-263716    Patent document 2: JP-A No. 2006-116388    Patent document 3: Japanese Patent No. 3382926    Patent document 4: PCT International Publication No. WO2008/139836